1. Field of the Invention
The invention pertains to a closing device in the form of an injection molding machine for plastics with a stationary mold clamping plate and with a movable mold clamping plate, which can be operated by a linear motor.
2. Description of the Related Art
A closing unit in which the end plate cooperates with the plunger to form a linear motor is known from DE 37 15 161 A1. The 3-phase windings are provided in the end plate and the plunger is designed as a xe2x80x9crotorxe2x80x9d rail.
The disadvantage of this design is the relatively weak closing force which can be exerted by the linear motor. In another embodiment, therefore, an electromagnet is provided to produce the required closing pressure; when current is flowing through this electromagnet, it induces a force on the plunger directed toward the mold.
The efficiency of linear motors is especially poor at low speeds and when no motion at all is occurring. In the design described in the previously cited document, however, the maximum demand for force occurs as the closing force is being built up. The only movement which occurs in this phase is that which results from the expansion of the tie bars. This leads to an uneconomically large installation cost for the linear motor and for the converter required to operate the drive.
It must also be expected that the amount of energy consumed will be uneconomical in comparison with rotating motors. This disadvantage cannot be avoided by the use of a divided end plate provided with electromagnets to build up the closing force. The expansion of the tie bars leads in this solution to a considerable widening of the air gap and thus to a drastic drop in the amount of force which can be transmitted. The only way to compensate for this is to increase the size of the magnets of the tie bar to an uneconomical extent.
In the proposed solution, furthermore, the closing force must continue to be actively applied during the injection, holding pressure, and cooling phases, which means that large amounts of energy must be consumed continuously.
An injection-molding machine is known from EP 0 280 743 B1, to which U.S. Pat. No. 4,895,505 corresponds, in which a linear motion element, here a metering/injection screw, is designed as a linear motor. The linear motor has a cylindrical form, which corresponds to a movable element with a circular cross section.
This design is called a solenoid motor and is used for applications with low force requirements. Solenoid motors achieve only a fraction of the force which can be generated by linear motors of the single-comb or multi-comb type.
The forces required to move the screw of an injection molding machine in a linear manner cannot be compared with the closing forces required to close the mold of an injection-molding machine; that is, they are typically are 5-10 times smaller. Because no additional rotational movements of the axle, which is moved in a linear fashion, are required to close the mold, a solenoid motor offers only disadvantages for this application.
A plastic injection-molding machine is known from DE 38 18 599 A1, in which at least some of the working elements are driven by arrangements of electrically superconducting magnets, the conductors of which are cooled by a coolant to a temperature below the transition point. One of these working elements is a toggle lever for closing the mold, for which electrically superconducting linear electromotors are provided for the linear motion.
This device suffers from the disadvantage that, to achieve the transition temperature required for the superconducting state, a conductor winding is required which consists of a special alloy, which must also be cooled. An alloy of this type consists in particular of the expensive metal lanthanum or yttrium, plus barium, copper, and oxygen. The transition temperature is reached in particular by the use of liquid nitrogen and is thus associated with significant technical effort.
The invention has the goal of creating a closing device of the general type in question which makes it possible by means of a simple design to move the movable mold clamping plate and to hold it in position at low expenditure of energy and without contamination.
According to the invention, at least one linear motor is connected to a force transmission element, which is connected in turn to a lever mechanism. This force transmission element can be designed as a crosshead or as an actuating frame.
Double-toggle levers are used here, where four-point and five-point toggle levers are preferred.
The geometry of the individual levers and the control program are selected so that, in the closed position of the movable mold clamping plate, the plate is held without any consumption of energy during the closed phase.
To achieve an especially short design, the linear motors are installed between the force transmission element and either the end plate or the movable mold clamping plate. In an especially advantageous design, the stationary part is attached to the end plate, and the mobile part is attached to the force transmission element. As a result, there is no need for a drag line.
In another advantageous design, a tie bar of the closing unit is used as the reaction rail of the linear motor.
During the closed phase of the movable mold clamping plate, the linear motors are turned off. As a result of this measure, the noise level is reduced and energy is also saved.
This is achieved by the use of toggle levers, which have such dimensions and are actuated in such a way via a control program that the dead center point is passed.
In another design, an arresting element is provided, which, during the closed phase, holds or grips at least one lever in a form-locking or friction-locking manner. The arresting elements are designed so that drive energy is required only for the locking and unlocking processes.
According to the invention, linear motors are used in a pairwise arrangement. As a result of this design, the gap forces are essentially compensated.
An essential advantage of the invention is that the linear motion is produced directly, without the need to convert a rotation into a translation by the use in particular of a gear transmission. The elimination of a transmission minimizes the maintenance work required and increases the reliability of the machine. Without a transmission, the drive operates without any hysteresis or clearance at all, which increases the precision and the controllability of the motion.
The principle of the linear drive imposes no limit of any kind on the speed or on the force of the motion, as is associated with a solution in the form of a transmission or a worm.